RIR, or reps in reserve, is the number of extra reps you could still have done when you ended a set. Stop a set of squats knowing two more reps were possible, and that set was 2 RIR. Zero RIR is failure. It's the most practical way to measure how hard a set actually was, regardless of the weight on the bar.
Why it matters
Muscle grows when sets get close to failure, but grinding every set to zero piles up fatigue fast and wrecks the quality of everything that comes after. RIR gives you the middle path. Research keeps showing that sets stopped 1 to 3 reps shy of failure build muscle about as well as sets taken all the way there, with far less wear on your joints and your recovery. So you keep most of the stimulus. And you keep training hard tomorrow.
How to use it in training
Run most working sets at 1 to 3 RIR. Program it directly: "3 sets of 8 at 2 RIR" means pick a weight where rep 8 leaves about two in the tank.
Calibrate first, though. Take one isolation exercise to true failure so you know where the wall really is. Most beginners stop 4 or 5 reps early while swearing they're close. Watch your bar speed too: when reps slow down noticeably, you're usually inside 3 RIR. As weeks go on, let RIR drop from 3 toward 1 before a deload. That's built-in progression.
Related terms
Go deeper
Curious what the studies say about failure vs stopping short? Read our full breakdown: Training to failure vs reps in reserve.